In recent years, aluminum electrolytic capacitors have been used in an application for inverters such as the wind power generation and solar power generation, and large-scale power sources such as batteries. In the aluminum electrolytic capacitors, hydrogen gas may be generated therein by a reverse voltage, overvoltage, and overcurrent, and there is a risk of rupture of an outer case by an increase in the internal pressure due to the generation of a large amount of hydrogen gas.
Therefore, ordinary aluminum electrolytic capacitors are provided with a safety valve having a special film. In addition to a function of releasing hydrogen gas in the inside of the capacitor to the outside, the safety valve has another function of preventing the rupture of the capacitor itself by self-destruction enabling to decrease the internal pressure of the capacitor when it is abruptly increased. As the special film that is a component of such a safety valve, for example, the following has been proposed.
Patent Document 1 has proposed a pressure regulator film equipped with a foil strip composed of a Pd—Ag alloy wherein 20 wt % (19.8 mol %) of Ag is incorporated into palladium.
However, the foil strip of Patent Document 1 has a problem that it tends to become brittle in an environment of about 50 to 60° C. or less and cannot maintain the function as a pressure regulator film for a long period of time. Thus, such a foil strip has not been put into practical use.
On the other hand, lithium-ion batteries are widely used in cellular phones, notebook computers, automobiles, or the like. Also in recent years, an interest in security for the lithium-ion batteries has grown in addition to higher capacity and improved cycle characteristics. In particular, gas generation in the cells of the lithium-ion batteries is known, and expansion and rupture of the battery pack accompanied with an internal pressure rise are concerned.
Patent Document 2 discloses use of an amorphous alloy (for example, 36Zr-64Ni alloy) composed of zirconium (Zr) and nickel (Ni) as a hydrogen permselective alloy film that selectively permeates hydrogen gas generated in the battery.
However, since the amorphous alloy becomes brittle due to the formation of a hydride compound (ZrH2) upon contact with hydrogen at a low temperature range (e.g. 50° C.), such an amorphous alloy had a problem that it could not maintain the function as a pressure regulator film for a long period of time.
In order to solve the above problem, Patent Document 3 has proposed a hydrogen-releasing film comprising a Pd—Ag alloy wherein the content of Ag in the Pd—Ag alloy is 20 mol % or more.
In Patent Document 4, a hydrogen-releasing film that includes a Pd—Cu alloy and has a Cu content of 30 mol % or more in the Pd—Cu alloy has been proposed to solve the above problem.
However, the hydrogen-releasing films of Patent Documents 3 and 4 are not easily embrittled at the use temperature of an electrochemical element and have a sufficient hydrogen-releasing performance at the beginning of use, but it has been a problem that the hydrogen-releasing performance gradually decreases depending on the use environment.